Downhole mud motors have been employed extensively in the drilling of wells, boreholes and other subterranean bores. One type of hydraulic downhole mud motor is the progressive cavity motor (or pump), which is also known as a Moineau motor (or pump). The progressive cavity downhole mud motor includes a power section that has a stator and a rotor disposed within the stator. The rotor rotates and gyrates in response to fluid (e.g., drilling fluid or drilling mud) pumped downhole and through the stator. A drive shaft is located within a bearing housing with the bearing housing being connected to the stator via a cross-over housing rigidly attached between them. A connecting rod extends between the rotor and the drive shaft for translating the rotation and gyration of the rotor to the true rotation of the shaft. The connecting rod may have an upset section on each end. Upper and lower connections connect the upset sections of the connecting rod to the rotor and to the drive shaft.
There are generally three types of connecting rods currently used in the industry. The first two types are similar and use two sets of articulating joints, one at each end of the connecting rod. The first of such type is generally known as a lobe coupling with surfaces between the upper and lower ends of the joint loading against two or more slotted surfaces that are cut radially to the center of the parts. Also, each upper and lower joint is loaded compressively along their longitudinal axis through a ball bearing or spherical surface against their respective spherical surfaces.
The second of such type using two sets of articulating joints is generally known as a universal joint or constant velocity joint. This joint consists of an internal and an external housing. The outside of the internal housing having spherical indentions that house several ball bearings, the external housing having radiused slots that ride over the outer half of each ball bearing. Again, each upper and lower joint is loaded compressively along their longitudinal axis through a ball bearing or spherical surface against their respective spherical surfaces. The articulated joints can be sealed from the circulating drilling mud using boots, O-rings or lip seals.
The third type of connecting rod is a flexible rod that is connected to both the rotor and drive shaft with rigidly attached connections normally of the rotary shouldered connection type.
Downhole mud motors have a cross-over housing connecting the stator to the bearing housing that locates the drive shaft relative to the stator. Motors of the steerable type have a bend or bends between the stator and bearing housing normally formed in the cross-over housing. The bend makes the bit offset to and at an angle from the center of the motor and bottom hole assembly (BHA) above. The steerable motor can drill straight ahead when the motor and BHA are rotated while fluid is pumped through the motor so that the offset load and bit angle are evenly distributed in all radial directions as the bit drills. Whenever the motor is needed to drill towards a specific direction, the drill string and BHA are stopped from rotating and located circumferentially in the direction desired. The drill string is slid so that the bit offset load and bit angle will cause the bit to drill towards the desired direction. Once the borehole has started in the desired direction the hole curvature is developed.
The size of hole curvature is controlled by the motor setup and BHA. For a more aggressive smaller curvature or faster direction change, the bend angle and offset are increased. This direction change is called a dogleg and is measured in degrees per hundred feet or per 30 meters. The bend angle is setup on a motor so that when a direction change is required, the borehole will be drilled at a minimum to the dogleg required. The dogleg is normally larger than required so that the borehole needs to be drilled straight for short distances to compensate. Through an extended direction change, the borehole is a series of short high doglegs with straight sections between. The optimum setup is to have as few of these changes as possible. Due to the configuration of the assembly, the motor, BHA and drill string must follow the bit through the curvature in the borehole. The size and geometry of the motor and BHA with respect to the borehole may restrict the passage or create large side loads on the motor and BHA when passing through the borehole. This is especially true as the curvature becomes smaller or changes direction more quickly with respect to distance drilled. It is advantageous to have the bend close to the bit so that there is not an excessive amount of offset at the bit for the amount of bit angle. When a motor is rotated in a curved section of the hole, the bit offset causes a high cyclic bending moment on the motor each time its bend rotates opposite the hole curvature. BHA studies and drilling experience have shown that there is a practical optimum range of bend to bit distance for each size motor for the amount of dogleg capability versus the bend angle. The practical shortest bit to bend distance is at the top of the bearing housing.
Lobe coupling articulated joints tend to wear at the load surfaces and crack at the base of the lobes. Constant velocity articulating joints tend to wear in the external housing slots and crack the ball bearings.
Due to the direction, or angle from centerline of the motor, of the connecting rod center rod, any thrust loads applied by the rotor through the connecting rod create side loads or radial loads at the rotor lower end and the drive shaft through the articulated joint. At the rotor lower end, increased thrust loads, such as from motor stall, cause the side loads to increase, pushing the rotor harder into the stator at the root of the stator lobe and away. The rotor tip is pulled away from the stator tip so that interference or elastomer squeeze in the stator is reduced and consequently the holding pressure and torque capacity is reduced.
A flexible connecting rod needs considerable length to have the torsional strength and diameter required to transfer the power section torque to the drive shaft and still have small enough side loads and bending loads so that the rotor connection, stator rubber and drive shaft connection have the load capacity to hold the loads. Even with a more flexible material for the rod, the lengths may become excessive due to increased power section capacities (in recent years, power sections have been introduced that generate very high torque, including “even-wall” stators such as the ERT series offered by Robbins & Myers, and hard rubber (HR) stators such as those offered by Dyna-Drill, where the higher torque results from the ability of these power sections to withstand higher operating pressures and pressure drops). As a result, connecting rods are required to be larger in diameter to handle the larger torque loads which increase their stiffness and which in turn requires more length to keep the side loads and bending loads from increasing.
Connecting rods with articulated joints on both ends allow the bend to be at or near the lower joint which is attached to the drive shaft. Conversely, the longer rigidly connected flexible rod is not well suited to have the bend at one end, because an already long rod must become larger and longer to overcome the increased bending moment at the lower end. For the flexible connecting rod with rigid connections, the smallest shortest rod has the cross-over housing bend situated half way between its ends. Also, an increase in bend angle creates an increase of the bending moment on such rod so that an even larger diameter and additional length is needed to keep the material stress levels below the endurance limit. Furthermore, this bending moment loads the rotor unevenly to one side in the stator at the lower end with the side load being towards the inside of the bend.
Nevertheless, the increased flexibility of such flexible connecting rod reduces the dynamic torques seen from sudden bit speed changes from hanging up or from slip stick. There is a tradeoff between rod flexibility and addition length of the motor required for that flexibility for both torsion and bending. The flexible rod is rigidly connected at both ends so that due to the offset a double bend is required in the rod and a direction change of the bending moment in the rod towards its center. There must be a side load at each end to create the moment at each end of the rod. For the rod to have the torsional strength required, it must be long enough from each end to reduce the side loads to an acceptable level and still create the bending moments required for the offset.
Connecting rods must be able to transfer the peak torques from the power sections to the bit. The peak torques can be as large as the motor stall torque plus the dynamic torque from bit hang up or slip stick.
The articulated joint at the lower end of the rotor gyrates with the rotor eccentrically about the stator at a rate of the number of rotor lobes times the motor rpm. This can create excessive centrifugal forces on the moving parts of an articulated joint. Also, the available diameter in this area that can be used for an articulating joint is reduced by the rotor sweep through the gyration from the eccentricity between the rotor and stator and the inside diameter of the stator tube or adjoining housing.
Due to more powerful motor torque capacities and diameter limitations, more efficient use of this space is needed. Tool joints have no moving parts with load surfaces that wear during the course of a motor run. They can more effectively transfer higher torque loads over the life of the tool.
However, threaded connections within the drive train also have torque limitations. The threaded connections can fail from their torque capacity being exceeded thereby causing the connection to make up further and yield the connection male and female members until it pulls itself apart or cracks and fails. Even with a tool joint connecting the rotor, there is a need to reduce the peak torque loads.
Various motor connecting rods and additional background information are disclosed in U.S. Pat. Nos. 4,636,151, 4,772,246, 4,982,801, 5,090,497, 5,267,905, 5,288,271, and 6,949,025, among others.